Several researchers have reported the successful cyclization of various aliphatic diamines to produce the corresponding heterocyclic imine compounds. In U.S. Pat. No. 2,952,688 a process for converting tetramethylene diamine to pyrrolidine using a supported nickel catalyst is disclosed. The compound is merely heated to a temperature of 100.degree. C.-160.degree. C. in the presence of the catalyst. Ammonia is liberated during the course of the reaction.
Later researchers have favored the use of vapor phase processes in order to avoid formation of oligomeric or polymeric by-products. For example, in U.S. Pat. No. 3,903,079 and U.S. Pat. No. 4,001,213, dehydrated crystalline aluminosilicate molecular sieve catalysts and metal or metal oxide catalysts respectively are disclosed. In the later reference nitrogen or hydrogen diluent gases are also employed in order to avoid polymeric by-products.
It has now surprisingly been found that relatively pure heterocyclic compounds may be prepared according to the instant invention by means of a liquid phase cyclization of aliphatic diamines without concomitant formation of polymeric by-products are predicted by prior art references. The ability to operate utilizing ordinary reaction containers and without the need for complex process equipment is considered an additional advantage.